Currently, a large majority of directional drilling is carried out in the smaller hole sizes, ie 8.5 inches or less (216 millimeters or less). In recent years, considerable interest in cost reduction and in increased productivity from marginal fields has led to a greater requirement for the drilling of high angle wells and horizontal wells. Additionally, the realisation that formation damage had a more significant effect on productivity than had previously been appreciated is causing a rapidly expanding interest in coiled tubing drilling, such that coiled tubing drilling has now overtaken slim hole drilling in respect of re-entry well work.
Control of direction when drilling is necessary but may be difficult, particularly in the smaller hole sizes. Direction control techniques available for larger hole sizes where the string is nominally rigid and can transmit high torque together with high longitudinal forces are not available for use in the relatively small diameter coiled tubing systems where the casings are flexible and cannot sustain high forces.